Biaxially-oriented poly(ethylene terephthalate) film base has a great many uses because it is rather inexpensive and yet is strong, tough, dimensionally-stable, and resistant to heat. However, most coatings do not adhere well unless the polyester film base first receives an adhesion-promoting treatment. There are a variety of such treatments, but for some coatings, no known treatment is fully effective.
Because of its low cost, a leading treatment is corona discharge even though it is only partially effective. Furthermore, because corona discharge treatment is transitory, any coating must be promptly applied to the treated polyester film base. Corona discharge treatment of poly(ethylene terephthalate) film is discussed in an article by Briggs et al.: "Surface Modification of Poly(ethylene Terephthalate) by Electrical Discharge Treatment", Polymer, 1980, Vol. 21, August, pages 895-900.
Where better adhesion is demanded, the polyester film base may be chemically treated, such as with para-chlorophenol as in U.S. Pat. No. 3,607,354, even though this involves a toxicity hazard. Organic priming or subbing coatings which are less toxic can also significantly enhance adhesion, and can be better adhered to the film base by subjecting the coating to ultraviolet irradiation as taught in U.S. Pat. Nos. 3,188,266 (Charbonneau et al.) and 4,210,703 (Scantlin et al.). Other adhesion-promoting treatments for polyester film base include spark and other electrical discharges, flame, and physical and/or chemical etching.
U.S. Pat. No. 2,955,953 (Graham) concerns promoting adhesion to any solid organic polymeric substrate by subjecting it in the substantial absence of oxygen to charged particle ionizing radiation having an energy of from 15 to 50,000 electron volts. The accelerated particles may be utilized in a vacuum or pass through a window and be utilized in air or a gas. In the Example, cellophane was irradiated in a cathode ray tube, and only the face of the film exposed to the electron beam exhibited the desired improvement in adhesion. The Graham patent suggests that not more than five minutes should elapse before a coating is applied unless the irradiated substrate "is kept in an inert atmosphere such as under nitrogen, argon, helium or the like and/or is stored at a low temperature such as at -80.degree. C." (column 2, lines 1-6).
Japanese patent application JA55-160598 (Takada et al.) which was laid open May 26, 1982 concerns the application by vacuum deposition of a magnetizable thin film such as Co/Ni to polyester film. Immediately before applying the magnetizable thin film, the polyester film base is placed in a poly(ethylene terephthalate) tube or bag and subjected to electron-beam radiation having an energy of 2.5 Mev (Examples 1 and 2). Although none of the examples give other conditions of the preirradiation, the final paragraph of the application states that the preirradiation may be conducted in a vacuum container, followed immediately by deposition of the magnetizable thin film to keep the surface of the film base free from moisture and dust. By doing so, the Co/Ni or other magnetizable film is said to adhere better to the polyester film base. See also the claim and the penultimate paragraph immediately preceding Example 1.
It is believed that any known adhesion-promoting treatment of polyester film base which is signficantly more effective than corona discharge also is significantly more expensive or involves hazards or both. Accordingly, the need has continued for a more effective treatment at a cost closer to that of corona discharge.
It is difficult to quantify the effectiveness of a treatment for promoting the adhesion of a coating to polyester film base. One test involves adhering the coating to an article to create a bond between the coating and article which is stronger than the bond between the coating and the film base. The force required to peel apart the film base and the article at 180.degree. is recorded as a measure of how well the coating is adhered. Although peel adhesion is meaningful, it may not precisely indicate the resistance of the coating layer to being removed when rubbed, such as when a magnetic recording layer is drawn across a magnetic recording head. A better indication of adequate adhesion to resist rubbing or abrasion failure has been obtained by testing the resistance of a coating to removal upon being scratched with one's fingernail or rubbed with one's thumb. The edge of a razor blade pressed vertically across a relatively moving coating can also provide a good indication of the resistance of the coating to removal when subjected to the sort of rubbing it may encounter in use.